Many materials, such as ceramic, are subjected to a heat treatment to attain desired characteristics thereof in the finished article. Accordingly, it is common practice to prepare test specimens of such materials to subject them to different heat treatments to accurately determine the effect of various heat treatments on the material. A number of methods and a variety of apparatus have been employed for preparing and studying such test specimens. In one such method, a series of identical test specimens are prepared, and each specimen is heat treated, in turn, at differing temperatures in a suitable furnace having means for varying the amount of heat applied; the heat attained during the firing period of each specimen is recorded and the series of fired specimens are studied to determine the one test specimen giving most nearly the desired results. Commercial production would then proceed upon the basis of firing to the temperature that was observed for the best of the test specimens.
It has been found that the above procedure many times is not adequate to determine the best specimen and the best firing temperature. For example, it is indeed possible that the optimum firing temperature may be missed if only a few specimens at widely varying temperatures are tested. Furthermore, with such methods it is difficult to determine the temperature actually attained in the specimen as distinguished from the temperature of the recording point in the furnace. This difficulty exists whether the heat treatment is in such a low range that measurement may be accomplished with mercury thermometers or in such a high range requiring platinum alloy thermocouples or the like as measuring devices. The reason for this difficulty is that there exists within the furnace and within the test specimen itself a temperature gradient in all directions, and the gradient will change in character as the maximum firing temperature is varied. Even though the measuring device may be applied directly to the face of the test specimen, there remains an element of uncertainty as to whether any part of the specimen is at a different temperature than is the surface. In addition, the measuring device may itself conduct away more or less heat depending upon its mass and conductivity, and thus influence the observed temperature at the measuring point. Another disadvantage of the abovedescribed test method is the relatively long time involved in separately testing a large series of specimens.
In an attempt to overcome the loss of time involved in separate testing of individual specimens, a method was developed which involved subjecting an elongated specimen to a varying gradient of temperature from a high value at one end, to a low value at the other end. This was accomplished in a so-called tube furnace, which in its common form comprises a ceramic tube of convenient length having wound thereon a heating coil of heavy duty electrical resistance wire, both the tube and winding being surrounded in turn by heavy thermal insulation to minimize any lateral heat loss. The effect of such an arrangement is to create a maximum temperature at the longitudinal center of the tube, with a gradual drop toward room temperatures at both exposed ends of the tube. In order to measure the temperature gradient, a thermocouple is mounted upon a rod which could be moved into the tube from above the specimen to hold the thermocouple at measured distances from the outer end long enough to obtain a stabilized reading. A temperature curve may be drawn from the several readings and then compared with the observed results of the firing along the length of the specimen. While this method assured a continuous record of firing results between the extremes of temperature from one end of the specimen to the other, there still remained a lack of assurance that observed temperatures corresponded with sufficient accuracy to those actually existing in the specimen at the various measured distances from the end. U.S. Pat. No. 2,825,222 to Stone attempted to overcome the problems associated with the latter method by providing apparatus for determining the actual temperatures in every portion of the elongated sample. Such apparatus is disclosed as including an elongated ceramic rod adapted to be placed in a tube-shaped furnace chamber. The elongated rod includes an upper surface shaped to closely engage specimens of materials placed thereon and further includes a series of temperature responsive elements mounted so as to be adjacent the upper surface of the rod.
Unfortunately, all of the prior art apparatus and techniques for preparing and studying test specimens subjected to different heat treatments suffer from the same fundamental disadvantages. This disadvantage resides in the fact that the series of specimens are not tested under identical conditions since the test specimens are not subjected to the same heat treatments at the same time. As a result, the test specimens produced do not accurately reflect the effects of different firing temperatures on the test specimens since many other variables effect the test specimens when they are tested at different times. For example, in the first prior art method discussed above wherein separate ceramic specimens are fired one at a time at different temperatures, the heat treatment to which each of the specimens is subjected have no common basis or reference for purposes of comparison as the specimens are fired at different times. In addition, each of these specimens are not heated in a heating zone or furnace which is heated to the same temperature throughout which also results in further varying of conditions and inaccurate test results. The Stone technique and apparatus is similar in this regard in that different areas of the heating zone or furnace are heated to different temperatures.
All heat treatment includes basically two factors, namely, the temperature at which a specimen is heated and the time that such specimen is heated at such temperature. The prior art technique wherein separate specimens are heated one at a time at different temperatures does not reflect the time that the samples are heated at each different temperature. The same holds true for the technique disclosed in the Stone patent.
The apparatus and method of the present invention overcome the disadvantages associated with the prior art apparatus and techniques and employs a furnace or heating zone which is heated at a uniform temperature throughout, and always contains the same number and/or mass of specimens so that each specimen removed from the furnace has been heated to all of the temperatures and under the same firing conditions to which the previously removed samples have been subjected.